The present invention relates to aerosol measurements, and more particularly, to instruments and techniques for characterizing cloud condensation nuclei.
Atmospheric particles influence the climate system, radiative transfer, visibility, and air quality. Hence, aerosol measurements of concentration, sizes, and chemistry of atmospheric particles are important in many applications, including monitoring air pollution and predicting climate change.
One aspect of aerosol measurements is characterization of cloud condensation nuclei (xe2x80x9cCCNxe2x80x9d). Under proper humidity conditions, certain aerosol particles are able to nucleate to form cloud droplets. Properties of cloud condensation nuclei provide important information on cloud formation and cloud properties. For example, cloud condensation nuclei can influence the droplet number and size distribution in a cloud, which ultimately affect a variety of processes including cloud lifetime and precipitation rate.
The ability of a particle to nucleate is at least in part determined by the saturation level of the environment, the size of the particle, and the chemical composition of the particle. For example, water vapor is more likely to condense on salt particles such as NaCl than on organic particles. When the relative humidity exceeds the saturation level where the vapor phase and the liquid phase are in equilibrium, a supersaturation state establishes and vapor begins to condense on surfaces and some particles to form droplets or condensation nuclei. At a certain critical supersaturation, when the diameter of a condensation nucleus of a given chemical composition exceeds a critical diameter, the nucleus is said to be xe2x80x9cactivatedxe2x80x9d, that is, vapor will condense spontaneously on that nucleus and cause the nucleus to grow to a very large size which is limited only by the kinetics of condensational growth and the amount of vapor available for the condensational growth.
The critical diameter at a given supersaturation usually changes with the chemical composition of the particles. Hence, particles of different chemical compositions can become activated at different sizes.
One way to characterize condensation nuclei is to measure the critical supersaturation at which a particle activates. Instruments for such measurements are generally referred to as cloud condensation nucleus counters. Cloud condensation nucleus spectrometers are such counters capable of producing and measuring supersaturations in a desired range. See, for example, Hudson, xe2x80x9cAn Instantaneous CCN Spectrometer,xe2x80x9d Journal of Atmospheric and Oceanic Technology, Vol. 6, p. 1055, December, 1989, and Hoppel et al., xe2x80x9cA Segmented Thermal Diffusion Chamber for Continuous Measurements of CN,xe2x80x9d Journal of Aerosol Science, Vol. 10, p. 369, 1979, which are incorporated herein by reference.
The atmospheric environment is usually dynamic. The activation and subsequent growth of could condensation nuclei originated from a subset of atmospheric aerosols are essential to formation of cloud droplets. Therefore, it is desirable to perform in situ measurements in order to accurately measure aerosol samples in real time and monitor the changing climate at a target location. A compact airborne cloud condensation nucleus spectrometer can be used to meet such demand. However, many conventional condensation nucleus spectrometers are ill-suited for small aircraft platforms due to limitations in various factors such as weight, size, time resolution, range of measurable supersaturation.
The present invention provides a novel CCN spectrometer which has been designed specifically for use on a remotely piloted aircraft for long periods of unattended operation, and which can measure CCN spectra over a wide range of supersaturation at high frequency (one spectrum per minute or faster). The instrument is also designed to be light and consume minimum power in order to conserve the limited resources available on small aircraft.
One embodiment of the CCN spectrometer implements a segmented cloud condensation nucleus growth column. A gas flow channel is formed within the column to receive and transfer a gas flow from an input opening to an output opening and having an inner wall which is wetted by a liquid. The segmented column has a plurality of alternating hot and cold temperature-maintaining segments arranged next to one another relative to the gas flow channel to control and maintain a temperature distribution along the gas flow channel. Each hot temperature-maintaining segment is maintained at a temperature higher than a cold temperature-maintaining segment. The temperatures produce a varying supersaturation environment within the gas flow channel.
In particular, a temperature difference between adjacent hot and cold temperature-maintaining segments increases from the input opening to the output opening to produce a supersaturation distribution that also increases from said input opening to said output opening.
A special optical particle counter is implemented to produce an optical probe beam to illuminate the gas flow in a close proximity to the output opening and to determine presence and dimension of particles in the gas flow.
These and other aspects and advantages of the present invention will become more apparent in light of the accompanying drawings, the detailed description, and the appended claims.